VCC East Campus, Spr '06, Dr. Mason
BSC1026: Biology of Human Reproduction
Fri. 9:30-11:45
Lecture Notes

Genes and Medical Genetics Outline Week 6
Ch. 2 in Human Reproductive Biology, pp. 22-39, Ch. 4 pp. 60-61

  • Genetics worksheet to be printed out and brought to class, Feb. 17th.
  • Necessary genetics vocabulary - p.22 and notes below
  • One-trait crosses - pp. 22-23, and notes below
  • Punnett square - p. 24, notes below and web link
  • Autosomal dominant and autosomal recessive disorders - pp. 26-27 and in-class genetics problems
  • X-linked recessive disorders - pp. 33-34 and in-class genetics problems
  • additional genetic types of inheritance, concepts and vocabulary - pp. 28-31, 36; see list at the bottom of posted notes and assignment #6.

Introduction to Genetics Problems

Necessary Definitions (some are repeats)

  1. somatic cells - all body cells except reproductive cells
  2. germ cells - reproductive cells that undergo meiosis
  3. gametes - products of meiosis that mature into eggs or sperms
  4. diploid (2n) - refers to cells that have pairs of chromosomes; all somatic cells in our body have 23 pairs: 2n = 46.
  5. haploid (n) - gametes have 1/2 the chromosome number of diploid cells, one chromosome of each pair -- 23 unique chromosomes: n = 23.
  6. homologous chromosome pair - each pair of our 23 pairs of chromosomes can be called an homologous pair; one chromosome came from dad's sperm and the other homolog from mom's egg. Together, each homologous pair of chromosomes has the same genes.
  7. autosomal chromosomes - 22 pairs of homologous chromosomes that women and men have; they are not our sex chromosomes
  8. sex chromosomes - one pair of chromosomes that differ between women and men. Women have 2 sex chromosomes that are a homologous pair called X chromosomes. Men a nonhomologous pair of sex chromosomes: one X chromosome that they always inherit from their mom and one small "male" chromosome called Y that they always inherit from dad.
  9. gene - instructional unit along the DNA ladder that codes for a protein.
  10. allele - alternate instructions of a gene; even though an homologous pair of chromosomes have the same genes, the specific instructions (alleles) of the genes may differ.
  11. phenotype - outward physical expression of a gene; "what you see"
  12. genotype - the allelic combination present on the homologous pair of chromosomes. (In other words, do both chromosomes of the homologous pair have the exact same allele or do the 2 alleles on the homologous pair differ in their instructional information?)
  13. dominant - the allele that is expressed phenotypically ("what you see") whether it is present on both chromosomes of the homologous pair or whether only one chromosome of the pair has the allele
  14. recessive - the allele that is only expressed phenotypically when it is present on BOTH chromosomes of the homologous pair (no dominant allele present)
  15. homozygous - the alleles for a gene on the homologous chromosome pair are the same
  16. heterozygous - the alleles for a gene on the homologous chromosome pair differ

 

When solving classical genetics problems for traits on autosomal chromosomes (chromosome pairs #1-22), a single letter abbreviation, upper and lower case is used to stand for the two alleles on the homologous chromosome pair. For example, the presence or absence of freckles is an inherited condition coded for by a single gene. In all our somatic cells we have 2 alleles for the freckles gene (on a homologous pair of chromosomes). Capital "F" stands for the dominanat allele whose genetic instructions result in the phenotype: make freckles. The lower case "f" stands for the recessive allele whose genetic instructions result in the phenotype: no freckles. Below are listed the genotypes (combinations of alleles), the phenotypes (outward expression that you would see) and the possible gametes for the freckles gene.

 

Genotype

Phenotype

Type of allele in the gamete

FF (homozygous dominant)

freckles

gamete with 'F' allele; instructions to make freckles


 Ff (heterozygous)


freckles

A heterozygous person can make 2 types of gametes: [1] a gamete with 'F' allele; instructions to make freckles or [2] a gamete with 'f' allele; instructions to make NO freckles

 ff (homozygous recessive)

no freckles

gamete with 'f' allele; instructions to make NO freckles

 

 

  • If you do not have freckles then you know your genotype. You must be homozygous recessive (ff).
  • If you do have freckles, then you may be one of 2 genotypes, homozygous dominant (FF) or heterozygous (Ff).
  • Go to this link: Baby Steps through the Punnett Square and read how to do Punnett squares. Also see Figure 2.4 and 2.5, p. 24 in your text. Then try to complete the Punnett square to solve the problem below.

Genetics Problem: Two freckled people both heterozygous have a child. What is the chance the child has no freckles?

 

 

 

 

X-linked Genetics Problems

  • The Y chromosome has only ~200 genes most of which are specific for determining the anatomy and physiology of a boy. We will not work genetics problems with these genes.
  • The X chromosome has some 1,200 genes that are totally unrelated to determining sex. The traits or diseases caused by recessive alleles of these genes are much more often displayed phenotypically in men than women since men have only one copy of the X. Also, since men inherit their X chromosome from mom, they always inherit the X-linked trait/disease from mom, not dad.
  • When solving genetics problems for genes on the X chromosome, we always write XX for women's genotype and XY for men's genotype and then assign a superscript letter on the X chromosomes to represent the gene.
  • Since women have 2 X's, they have 2 alleles for the genes on the X chromosomes; they can be homozygous recessive, homozygous dominant or heterozygous. See the table below.
  • Since men just have one X chromosome, they either have the dominant or the recessive allele. The genotype terminology above does not apply. Complete the table below:

 

genotype

phenotype

gametes

XBXB
homozygous dominant

female; see color

1 egg type
with XB allele

XBXb
heterozygous

female; see color

 2 types of eggs:

XbXb
homozygous recessive

1 egg type:

  XBY

2 types of sperms,
XB or Y

XbY

  2 types of sperms:
 

 

Solving Genetics Problems

  • We will spend all of Friday's class, after the test, solving genetics problems. Make sure you print out the genetics problems worksheet and bring it to class! By the end of class you should be able to solve single gene genetics problems of 3 types:
     

  1. autosomal recessive - the trait or disease of interest is coded for by a recessive allele so people who display the trait or disease have the genotype of homozygous recessive (aa)
  2. autosomal dominant - the trait or disease of interest is coded for by a dominant allele so people who display the trait or disease are homozygous dominant or heterozygous (AA or Aa)
  3. X-linked recessive - the trait or disease is coded for by a recessive allele on the X chromosome. Women
    who display the trait or disease are homozygous recessive, XaXa , and men with the trait or disease are XaY.

 

Specific concepts to know from chapter 2:

(As part of your homework, you will read in your textbook and answer questions about the following types of inheritance in assignment #6.)

  • polygenic inheritance and an example: height, hair color, skin color
  • multifactorial disorders and examples: diabetes, hypertension, cancer, many behavioral traits
  • multiple alleles and ABO blood typing; codominance of AB
  • pleiotropy and an example: Marfan syndrome
  • incomplete dominance and an example: straight, wavy, curly hair
  • sex-influenced traits and an example: baldness
  • mitochondrial inheritance: always from mom